1. Field of the Invention
The present invention relates to a thermal transfer recording apparatus for recording images on a recording medium by transferring ink contained in an ink sheet to the recording medium, and a facsimile apparatus using the aforesaid apparatus.
2. Related Background Art
In general, a thermal transfer printer uses an ink sheet with a thermally fusible (or thermally sublimable) ink coated on the base film thereof, and selectively heats such ink sheet by the thermal head in response to image signals in order to transfer the fused (or sublimated) ink to a recording sheet for image recording. Usually, an ink sheet of the kind is such that the contained ink is completely transferred to the recording sheet for one image recording (the so-called one-time sheet). Therefore, it is necessary to convey the ink sheet for an amount equivalent to the length of one recorded character or one line of image after the image recording has been completed, so that the unused portion of the ink sheet can reliably be brought forward to the position for the next recording. Thus the consumption of the ink sheet becomes great and the running cost of the thermal transfer printer tends to be higher than that of a usual thermal printer using thermal sheets for recording.
With a view to solving a problem such as this, there has been proposed a thermal transfer printer in which a recording sheet and an ink sheet are conveyed with a difference in the conveying speeds thereof such as disclosed in Japanese Patent Laid-Open Patent Application No. 57-83471, Japanese Patent Laid-Open Application No. 58-201686, and Japanese Patent Publication No. 62-58917.
The present invention is designed to make a further development of those patents disclosed in the aforesaid applications or publication.
As the ink sheet to be used for these thermal transfer printers, there is known an ink sheet (multiprint sheet) capable of recording images plural times (n). Then, when a length L of recording is continuously performed using this ink sheet, it is possible to carry on such a recording by making the length of the ink sheet conveyed after each image recording has been completed or while the image is being recorded shorter than the length L by (L/n:n&gt;1). Hence, the efficiency with which the ink sheet can be used becomes more than the conventional case by n times, and a reduction of the running cost of the thermal transfer printer can be expected. Hereinafter, this recording method is referred to as multiprint.
In the case of a multiprint such as this, the ink layer of the ink sheet is heated by n times separately. Then, at each time of heating, the ink transfer to the recording sheet is performed by generating a shearing force between the fused ink on the ink layer and the ink yet to be fused thereon. As a result, if, for example, the temperature of ink is lowered due to a longer period of time having elapsed before the recording of the next line subsequent to the recording of one line, the force required to shear the fused ink and the ink yet to be fused (sublimated) becomes greater, leading to a problem that the ink sheet and recording sheet are not easily separable. This condition is particularly conspicuous when a one line recording data contains more black information, and in a facsimile apparatus or the like in which a time interval between a current line and the next line is not constant and such time intervals tend to be comparatively long so, this condition presents a problem.
In other words, when black information is contained in a considerable amount, the force required to shear between ink layers becomes great if the recording cycle is long because the ink which has once been fused is cooled and this phenomenon appears as an adhesion of the ink sheet to the recording sheet. Therefore, there is a possibility that if a predetermined number of lines should be recorded subsequent to having recorded a line information containing higher black dot numbers, an adhesion such as this occurs depending on its recording cycle and black data rate in one line. If such adhesive force is increased, an elongation of the ink sheet, deflection of the rubber of the platen roller, and the like are generated, making the velocities of the ink sheet and recording sheet essentially equal. Hence, the relative velocities thereof are brought into a level close to substantially zero and there is a possibility that the recording quality is lowered due to the multiprint thus applied.